What is the minimum number of NGS First or Second Order benchmarks required to establish the Elevation/Vertical control network?

What is the minimum number of NGS First or Second Order benchmarks required to establish the Elevation/Vertical control network?

• A. One
• B. Three
• C. Two
• D. Four

Answer: (C)

Vertical control networks must be anchored to known elevations to define and verify the local datum. A single benchmark provides a reference elevation for one point, but there’s no way to fix the overall height reference or test for consistency across the network. With two benchmarks, you have two known elevations, which lets you determine the height difference between the anchors, anchor the vertical datum at two points, and propagate elevations to other points while checking for closure and consistency. This redundancy is the minimum needed to establish a reliable Elevation/Vertical control network, hence two First or Second Order benchmarks.

Vertical control networks must be anchored to known elevations to define and verify the local datum. A single benchmark provides a reference elevation for one point, but there’s no way to fix the overall height reference or test for consistency across the network. With two benchmarks, you have two known elevations, which lets you determine the height difference between the anchors, anchor the vertical datum at two points, and propagate elevations to other points while checking for closure and consistency. This redundancy is the minimum needed to establish a reliable Elevation/Vertical control network, hence two First or Second Order benchmarks.